The present invention pertains to the art of manufacturing extrudable articles, and particularly thin-walled honeycomb structures from extrudable ceramic batches which have the property of being able to flow or plastically deform under pressure during extrusion, while being able to maintain their as-extruded form under ambient conditions after being relieved of the high extrusion force. More particularly, the present invention relates to an improved method of conditioning and/or refining extrudable ceramic batch material by purifying and homogenizing the same while it is subjected to extrusion pressures during the operation of extruding a finished product, but prior to such material being extruded through a discharge die for forming the finished article.
In the extrusion forming of articles from ceramic material, it has been common practice to pre-extrude the ceramic batch material through a pre-extrusion die prior to the final extrusion process, in order to further homogenize and de-air the batch. Such a pre-extrusion die is provided with a plurality of bores or passages extending therethrough for forming strands or "noodles" of batch material which is subsequently re-extruded into a final article. In fact, the pre-extrusion die is also known in the trade as a "noodle" die or "spaghetti" die due to the noodle or spaghetti-like shape of the batch as it is discharged therefrom.
When utilizing a piston and cylinder type of extrusion chamber, as opposed to a screw feed extrusion chamber utilized with organic plastic materials, the extruding process by necessity becomes one of a "batch" operation. That is, the piston or plunger is extracted, the cylinder filled with previously pre-extruded spaghetti batch material, and the plunger then moved forwardly within the cylinder, after adequate evacuation thereof to prevent voids, so as to extrude the thus charged batch material outwardly through the article-forming die on the discharge end of the extrusion cylinder. When the plunger has completed its travel, it is again withdrawn from the cylinder, the cylinder again recharged with a new supply of previously pre-extruded spaghetti batch material, and after suitable evacuation of the cylinder the plunger is again moved forwardly to recommence the extrusion of the newly charged material through the article-forming die. This batch process is continuously repeated until article requirements are fullfilled.
It has been found that by compacting the pre-extruded noodle batch material into billets of cylindrical shape having a cross section substantially equal to the cross section of the extrusion cylinder, a greater quantity of batch can be loaded into the cylinder and thereby increase the output from the extrusion die per plunger travel. The compacted batch material or billets are also extruded as a batch operation similar to that previously described relative to the spaghetti batch material. However, when utilizing compacted batch in billet form it was found that an interface was formed between adjacent billets which was carried into the extrusion die resulting in problems relating to the formation of voids and torn skins in the finished article, particularly when extruding honeycomb articles having thin walls forming longitudinal passages. Similar types of interface problems were also encountered between succeeding charges of the spaghetti batch material.
The present invention overcomes the problems encountered in the prior art relative to the formation of interfaces between adjacent batch material which tend to result in voids and skin peel-off in the finished product, by homogenizing the batch material within the extrusion cylinder during the extrusion operation but prior to being forced through the article-forming extrusion die. To accomplish this end, after the billets of ceramic batch material have been loaded into the extrusion cylinder, the cylinder is evacuated and the plunger moved forwardly to force the billets through a homogenizer positioned a given distance upstream of the extrusion die. The homogenizer not only screens out unwanted large particles of particulate matter in the batch material but also breaks-up and homogenizes the interfaces between adjacent billets by again forming the billets into strands, and then compacting the strands or spaghetti into a continuous slug in a bottom section of the flow homogenizer with the continuous slug eliminating all of the previously enumerated transition and interface problems. The continuous slug is then extruded through the article-forming die, thereby providing such die with a homogeneous batch having no batch interfacial discontinuities.